1. Field of the Invention
The present invention relates to a method for making a lithographic printing plate wherein an image-wise exposed precursor is developed with an alkaline developing solution, an anti-sludge agent is present in the precursor or in the developing solution or in both the precursor and the developing solution, and the anti-sludge agent is a 5-membered heteroaromatic compound, including a —NH— group wherein the hydrogen is capable of being deprotonated in the alkaline developing solution, selected from the group consisting of an optionally substituted benztriazole, 1,2,3-triazole, tetrazole, or indazole compound.
The present invention relates also to an alkaline developing or replenishing solution including the anti-sludge agent.
The present invention relates also to a lithographic printing plate precursor including the anti-sludge agent.
2. Description of the Related Art
Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called a plate precursor. A typical positive-working plate precursor includes a hydrophilic support and an oleophilic coating which is not readily soluble in an aqueous alkaline developer in the non-exposed state and becomes soluble in the developer after exposure to radiation. In addition to the well known photosensitive imaging materials which are suitable for UV contact exposure through a film mask (the so-called pre-sensitized plates), heat-sensitive printing plate precursors have also become very popular. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method (CtP) wherein the plate precursor is directly exposed, i.e., without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, and solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer.
Although some of these thermal processes enable plate making without wet processing, the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically includes an oleophilic binder of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure.
Typically, the oleophilic resin in a heat-sensitive plate is a phenolic resin such as novolac, resol or a polyvinylphenolic resin. The phenolic resin can be chemically modified whereby the phenolic monomeric unit is substituted by a group such as described in WO 99/01795, EP 934 822, EP 1 072 432, U.S. Pat. No. 3,929,488, WO 2004/035687, WO 2004/035686, WO 2004/035645, WO 2004/035310. The phenolic resin can also be mixed with other polymers as described in WO 2004/020484, U.S. Pat. No. 6,143,464, WO 2001/09682, EP 933 682, WO 99/63407, WO 2002/53626, EP 1 433 594, and EP 1 439 058. The coating can also be composed of two or more layers, each of them including one or more of the above described resins as described in, e.g., EP 864420, EP 909657, EP-A 1011970, EP-A 1263590, EP-A 1268660, EP-A 1072432, EP-A 1120246, EP-A 1303399, EP-A 1311394, EP-A 1211065, EP-A 1368413, EP-A 1241003, EP-A 1299238, EP-A 1262318, EP-A 1275498, EP-A 1291172, WO 2003/74287, WO 2004/33206, EP-A 1433594, and EP-A 1439058.
For a positive-working thermal plate, a dissolution inhibitor is added to a phenolic resin as a binder whereby the rate of dissolution of the coating is reduced. Upon heating, this reduced rate of dissolution of the coating is increased on the exposed areas compared with the non-exposed areas, resulting in a sufficient difference in solubility of the coating after image-wise recording by heat or IR-radiation. Many different dissolution inhibitors are known and disclosed in the literature, such as organic compounds having an aromatic group and a hydrogen bonding site or polymers or surfactants including siloxane or fluoroalkyl units.
During processing, the solubility differential leads to the removal of non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while image (printing) areas of the coating remain on the support. Thereby, the coating of the non-image (non-printing) areas is solubilized in the alkaline developing solution and the ingredients constituting the coating cumulate in the developing solution and may result in the formation of sludge. Sludge is to be understood as all kinds of insoluble matter such as undissolved, flocculated or precipitated products or gel structures, formed in the developing solution by interaction between the ingredients of the coating and the developing solution. This insoluble matter may adhere on the plate surface impairing the image on the plate and may also contaminate the developing station by adhesion on the surface of the rollers and the developing tanks. As a result of the formation of sludge, the development process may be hindered (by increased viscosity of the developing solution, impairing of the image on the plate and contamination of the developing station, clogging of filters) and the developing solution needs to be changed regularly.
WO 97/39894 discloses an oleophilic heat-sensitive composition including a polymer, soluble in an aqueous developer, and an inhibiting compound such as a nitrogen-containing heterocyclic compound (e.g., quinoline and 1,2,4-triazole), which reduces the aqueous solubility of the polymer, and wherein the solubility of the composition in the developer is increased upon heating.